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Well it comes with a magnetic foam pad but I'm using my lap desk thing for laptops because it has a built in wrist pad for the keyboard. Here's a link to the amazon page where I got it: https://www.amazon.com/dp/B017EVRZG4/ref=cm_sw_r_cp_api_dqYaybG9F1QGM
Edit: I thought you meant what I was using to secure my Lapdog. As the other guy said the Lapdog by Corsair is what I'm using to actually mount my keyboard and put my mouse on
I went with a XL Lapdesk it fits the 17R4 and there is a nice slot to hold cell phone as well. Only complaint is the beans in the bottom pads wear out quick.
LapGear XL Deluxe Laptop LapDesk 91498 Black Carbon https://www.amazon.com/dp/B017EVRZG4/ref=cm_sw_r_cp_tai_dsS6ybM5473W6
I have one of these and love it-
Not foldable so not sure if it will meet your needs. I use it with a 17" laptop and mouse. I have full range of motion with the mouse and the trackpad works very well. The pillows on the bottom are comfortable and I really like the little slot on the top right where I keep my phone and two flash drives.
I use this one and it fits fine
That's actually how I did it before i got the Lapdog. Here's the one I used: https://www.amazon.com/dp/B017EVRZG4/ref=cm_sw_r_cp_api_dqYaybG9F1QGM